We have characterized the mechanisms by which thrombin enhances neutrophil leukocyte (PMN) adhesion to human endothelial cells in vitro. Thrombin rapidly and transiently increased PMN adhesion by an action on the endothelial cells. The transience of the response was due to at least two factors: desensitization of the endothelial cell responsiveness to thrombin in the continued presence of the agonist; and the lability (t1/2 less than 15 min) of the effector molecules expressed by the endothelium. Experiments with exogenous platelet-activating factor (PAF) and with PAF antagonists demonstrated that PAF production, although it may facilitate the enhanced PMN adhesion seen in response to thrombin, is not sufficient to explain the reaction. By using a variety of antibodies directed against cell surface ligands, and comparing adhesion of PMN to endothelium and to protein-coated surfaces, we deduce that several endothelial ligands not previously reported as playing a role in PMN adhesion are involved in these interactions. Of particular interest was the finding that antibodies recognizing two thrombin-regulated endothelial cell surface ligands, GMP-140 and the CD63-related Ag, both inhibited adhesion of PMN to thrombin- or LPS-pretreated endothelium. We conclude that thrombin acts to enhance PMN adhesion to endothelium at least in part by transiently altering the conformation or level of expression of these ligands.